Tunable Exchange Bias Effects

Abstract

Extrinsic control mechanisms of the interface magnetization in exchange bias heterostructures are reviewed. Experimental progress in the realization of adjustable exchange bias is discussed with special emphasis on electrically tunable exchange bias fields in magnetic thin film heterostructures. Current experimental attempts and concepts of electrically controlled exchange bias exploit magnetic bilayer structures where a ferromagnetic top electrode is in close proximity of magnetoelectric antiferromagnets, multiferroic pinning layers, or piezoelectric thin films. Various experimental approaches are introduced and the potential use of electrically controlled exchange bias in spintronic applications is briefly outlined. In addition, isothermal magnetic field tuning of exchange bias fields and extrinsically tailored exchange bias training effects are reported. The latter have been studied in a variety of systems ranging from conventional antiferromagnetic/ferromagnetic bilayers and core–shell nanoparticles to all ferromagnetic heterostructures where soft and hard ferromagnetic thin films are exchange coupled across a non-magnetic spacer. Such ferromagnetic bilayers show remarkable analogies to conventional exchange bias systems. At the same time they have the experimental advantage to provide direct access to the magnetic state of the pinning layer by simple magnetometry. A large number of exchange-coupled magnetic systems with qualitative differences in materials composition and coupling share a common physical principle that gives rise to training or aging phenomena in a unifying framework. Deviations from the equilibrium spin configuration of the pinning layer generate a force that drives the system back toward equilibrium. The initial nonequilibrium states can be tuned by temperature and applied set fields providing control over various characteristics of the training effect ranging from enhancement to complete quenching.